106 



MICROTIN^ 



The unworn w^j of Dicrostonyx (Fig. 57) shows no fewer than 

 seventeen more or less distinct tubercles arranged in three longi- 

 tudinal rows. Although these tubercles are but feebly salient, 

 they are clearly recognizable upon careful microscopic examina- 

 tion ; those capping the five posterior closed triangles are the best 

 developed, but those capping the anterior loop can scarcely be 



Pig. 67. — Dicrostonyx grcenlandicus Traill, 



Right m^ and left Wj of a new-born individual, unworn. The sketch at 

 the bottom is a camera lucida drawing of part of the m^ viewed obliquely 

 from the outer side and shows the tubercles capping the posterior loop 

 and five posterior triangles (B.M., No. 78.5.13.1, Floeberg Beach, Grinnell 

 Land; lat. 82° 27' N., July 1876, Capt. Feilden). 



said to be differentiated from each other. The tubercles of the 

 median row are relatively large ; each of them crowns fully half 

 of one of the inner triangles together with the corresponding central 

 portion of the tooth ; the extremity of each inner triangle is 

 crowned by an inner tubercle ; an outer tubercle caps each 

 outer triangle. The posterior loop is capped by three tubercles, 

 namely, a small outer (cusp 6), a large inner (1), which is probably 

 a compound of several distinct elements, and a small antero- 



